In producing long coated optical fibers, care must be taken to ensure that the coated optical fiber or buffered optical fiber is not broken during the production thereof. The glass fibers are comprised of glass material which is broken by fine failures or cracks formed therein. Accordingly, when producing long coated optical fibers it is desirable to prevent such fine failures or cracks from developing in the glass over an extended length.
To eliminate this problem there has been proposed a method in which coated optical fibers having a predetermined length are first produced in place of a long coated optical fiber. Thereafter, the fibers are spliced to each other and mold-reinforced at the spliced parts to provide a long optical fiber.
As shown in FIG. 1, a coated optical fiber 1 has a two layer-coated structure. More specifically the coated fiber is comprised of a bare glass fiber 2 covered with a primary coating 3 (e.g., silicone). The coating 3 provides protection, reinforcement, and so forth of bare glass fiber 2 providing a buffered optical fibers 4. Furthermore, the buffered optical fiber 4 is covered with a secondary coating 5 (e.g., nylon).
When bare glass fibers 2--2 are exposed and spliced to each other and, thereafter, are molded at the spliced part by the above-described method, the mold reproduced-part 6 has a single layer-coated structure. Problems result when the long coated optical fiber is bent, in that the bending stress is concentrated at the molded boundary to a change in Young's modulus resulting from the difference in structure. Moreover, when the spliced part is subjected to a surface treatment using, for example, hydrofluoric acid for the purpose of improving the strength thereof, the hydrofluoric acid penetrates to the area between the coated optical fiber 1 and buffered optical fiber 4. When this happens it is necessary to remove the hydrofluoric acid, and if the hydrofluoric acid is insufficiently removed and remains in the inside, it causes a reduction in the strength of the coated optical fiber. Therefore, the long coated optical fiber produced by the above-described method is inferior with respect to reliability over a long period of time.